Such an arrangement with often a plurality of, but at least two, electrical current sources which are connected to a common load via the switching units can be provided, for example, in a photovoltaic (PV) system. The individual current sources are in this case parts or regions of the photovoltaic (PV) generator, referred to below as PV subgenerators, which are connected to a DC input circuit of an inverter as common load. In such PV systems, isolation, under certain circumstances also galvanic isolation or even galvanic isolation at all poles, is generally required between the PV subgenerators and the inverter, which isolation is implemented by an electromechanical switch. An electromechanical switch is understood to mean an electromechanically actuated switch, actuated by an electric motor or an electromagnet, for example. Switches actuated by an electromagnet are also referred to as relays or contactors.
In order to prevent switching on load of these electromechanical switches resulting in the formation of an arc at the switching contacts of the switches, semiconductor switches are often used in parallel with the electromechanical switches. The semiconductor switches bypass the electromechanical switch temporarily during the switching operation and thus prevent the formation of an arc. Such an arrangement of a switching unit with an electromechanically actuated switch and a semiconductor switch arranged in parallel therewith is known, for example, from the document DE 10 2008 057 874 A1. Starting from a closed electromechanical switch and an open semiconductor switch, in order to disconnect a PV subgenerator from the inverter, for example, first the semiconductor switch is closed and then the electromechanical switch is opened, wherein an arc is prevented since there is no or only a small potential difference between the switching contacts. Finally, the semiconductor switch is opened.
During operation of a PV system, operating states can occur, in which it is desirable for all of the PV subgenerators to be disconnected, also called isolated, from the input of the inverter successively as quickly as possible. Such a situation can arise, for example, when an arc detector arranged in the load circuit indicates the presence of an arc in one of the PV subgenerators. In particular in the case of large PV systems, simultaneously decoupling of the entire PV generator, i.e. simultaneous decoupling of all of the PV subgenerators, is undesirable since instabilities can occur in the power supply system to which the inverter feeds. In addition, subsequent faultfinding is complex since, in the case of such simultaneous disconnection of all of the PV subgenerators, no localization of the arc can take place.
If, on the other hand, the PV subgenerators are isolated from the DC input circuit of the inverter successively, the location of the arc can be restricted at least to the level of the PV subgenerators. Such a method for localizing an arc is described in the document DE 101 55 795 C1 in connection with a motor vehicle power supply system that has a plurality of subsystems. Owing to the inertia of the electromechanical switches, via which the PV subgenerators are connected to the inverter, a single switching operation does have, however, a minimum switching duration which can be in the region of a few tens of milliseconds. In the case of large PV systems, with a correspondingly large number of PV subgenerators, sequential disconnection can then take a few seconds, under certain circumstances. In the most unfavorable case, the arc remains until disconnection of the last PV subgenerator, i.e. over the entire duration of the switching sequences. This is undesirable for safety reasons, for example owing to a fire hazard originating from an arc, and is also impermissible under certain circumstances, for example in accordance with the US standard UL1699B.
In other operating states, however, it may be desirable to connect PV subgenerators to an inverter sequentially as quickly as possible.
Some aspects of the present disclosure provide methods for disconnecting current sources, for example PV subgenerators, from a common load, for example an inverter, sequentially and as quickly as possible or connecting the current sources to this common load sequentially and as quickly as possible.
This aspect can be achieved by the methods for sequential disconnection or sequential connection comprising the features of the independent claims. Advantageous configurations are given in the dependent claims.
A method of sequential disconnection according to the disclosure of the type mentioned at the outset comprises the following steps: Semiconductor switches that are associated with a closed electromechanical switch are closed if they are not already closed and then the respective electromechanical switches are opened. In a preferred embodiment of the invention the opening of the respective electromechanical switches takes place simultaneously or at least nearly simultaneously. Then, at least two of the semiconductor switches that were actuated in the first step or that were already closed are opened sequentially.
A method according to the invention for sequential connection of the type mentioned at the outset comprises the following steps: Semiconductor switches which are associated with a closed electromechanical switch are closed if they are not already closed, and then the respective electromechanical switches are opened. In a preferred embodiment of the disclosure the opening of the respective electromechanical switches takes place simultaneously or at least nearly simultaneously. Then, a plurality of the semiconductor switches are opened, of which then at least two semiconductor switches are in turn closed sequentially.
Both methods are based on the basic concept of using the semiconductor switches for implementing the switching sequence, i.e. for successive (sequential) connection of the load to or disconnection (isolation) of the load from the individual current sources, instead of the electromechanical switches. For this purpose, first the switching state of the electromechanical switches is transferred to the associated semiconductor switches. These can then be opened successively quickly or first be opened jointly and closed quickly successively. The semiconductor switches which are generally arranged in parallel with an electromechanical switch as protection against a switching arc can thus be used to disconnect or connect a load from or to a plurality of current sources quickly and sequentially.
In an advantageous embodiment of the method, a semiconductor switch actuated during the sequential opening or sequential closing remains opened or closed until all of the semiconductor switches to be actuated sequentially have been actuated. In a preferred embodiment, after completion of a sequential closing of the semiconductor switches, the associated electromechanically actuated switches are closed.
Alternatively, a semiconductor switch actuated during the sequential opening or sequential closing is first closed or opened again before a next semiconductor switch is actuated sequentially.
In a further advantageous embodiment of the method, the sequential opening or the sequential closing of the semiconductor switches is characterized by an opening or closing of in each case an individual one of the semiconductor switches. Alternatively, the sequential opening or the sequential closing of the semiconductor switches is characterized by a simultaneous opening or simultaneous closing of in each case a plurality of the semiconductor switches. Thus, switches can be operated individually or in groups comprising two or more individual switches.
In a further advantageous embodiment of the method, after completion of the sequential opening or sequential closing of the semiconductor switches, the following additional steps are performed:
A subset of current sources that are intended to be connected to the common load again is established.
All of the semiconductor switches of the current sources which belong to the subset if the semiconductor switches are not already closed, are closed. Then, all of the electromechanically actuated switches of the current sources which belong to the subset are closed. It is preferred that the current sources that are not intended to be connected to the common load again, are galvanically isolated from the load.
In accordance with a further aspect of the disclosure, the above-described method is used for localizing and quenching an arc in a PV generator in a PV system, wherein the PV generator comprises at least two PV subgenerators as current sources that are connected, via switching units, to a DC input of an inverter as common load.
In accordance with a further aspect of the disclosure, the above-described method is used for determining partial currents in a PV generator in a PV system, wherein the PV generator comprises at least two PV subgenerators as current sources, which are connected, via switching units, to a DC input of an inverter as common load, wherein a current measurement device for measuring a current through the load is provided.
The above-described methods for sequentially disconnecting or connecting individual current or voltage sources from or to a common load can also be used to localize a possible insulation fault within a plurality of PV subgenerators connected in parallel quickly, as a means of determining the insulation resistance (RINS measurement) of a PV generator. The faulty PV subgenerator can then be galvanically isolated permanently after identification of an insulation fault. Equally, it is conceivable to determine discharge currents and the discharge capacitances causing the discharge currents of individual PV subgenerators quickly using only one (centrally arranged) measurement system.
In the case of PV systems, switching units via which PV subgenerators can be connected to an inverter generally comprise a parallel circuit comprising an electromechanical switching element and a semiconductor switch that protects the contact path of the electromechanical switching element from a switching arc. The method according to the disclosure can advantageously be used for being able to localize a detected arc quickly. The quick switching sequence also makes it possible to implement a multiplexing method for an individual measurement of partial currents in individual PV subgenerators using only one current measurement device without any significant losses in yield.